Random access memory (RAM) devices permit execution of both read and write operations on memory cells to manipulate and access stored binary data or binary operating states. Exemplary RAM devices include dynamic random access memory (DRAM) and static random access memory (SRAM). Typically, a high binary operating state (i.e., high logic level) is approximately equal to the power supply voltage and a low binary operating state (i.e., a low logic level) is approximately equal to a reference voltage, usually ground potential. SRAM memory cells are designed to hold a stored binary operating state until the held value is overwritten by a new value or until power is lost. In contrast, DRAM memory cells lose a stored binary operating state unless periodically refreshed every few milliseconds by sensing the held value and writing that held value back to the DRAM cell thereby restoring the DRAM memory cell to its original state. Memory circuits composed of DRAM memory cells are favored in many applications, despite this limitation, over memory circuits based upon SRAM memory cells because of the significantly greater attainable cell densities and low power required.
The area required for each SRAM memory cell contributes to determining the data storage capacity of an SRAM memory circuit. This area is a function of the number of elements constituting each memory cell and the feature size of each element. Conventional SRAM memory cells consist of four to six transistors having four cross-coupled transistors or two transistors and two resistors, as well as two cell-access transistors. A DRAM memory cell may be fabricated with a single capacitor for holding a charge and a single transistor for accessing the held value stored as charge in the capacitor, in contrast to the numerous transistors required for each SRAM memory cell. Absolute SRAM cell size can be improved with reductions in feature size arising from advances in lithography technology. However, further reductions in SRAM cell size may require more radical changes to the basic cell configuration. Despite their advantages over DRAM cells, conventional SRAM cells are expensive to produce and consume large areas on the substrate surface, which limits cell density.
The operation of a gain cell contrasts with the operation of both SRAM cells and DRAM cells. In a conventional gain cell, charge held by a storage capacitor operates as a gate that regulates current sensed over sense source and sense drain lines by remote access circuitry. Similar to a DRAM cell, the held values of a gain cell must be periodically refreshed. Although gain cells are less compact than DRAM cells, gain cells operate faster than DRAM cells. Although gain cells operate slower than SRAM cells, gain cells are more compact than SRAM cells. Therefore, gain cells are suitable candidates for applications such as on-chip cache memories.
What is needed, therefore, is a memory circuit in which each gain cell consumes less area per cell than conventional SRAM cells, incorporates a storage capacitor as a storage device, and features simplified access requirements.